configs: add defconfig for Atmel AT91SAM9260-EK Nand Flash Boot

This is a configuration that provides a basic setup for generating
bootable nandflash images:
- at91bootstrap
- barebox
- kernel
- rootfs

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>